Various types of operator windows are known in which a sash is movable relative to a perimeter frame between respective open and closed positions of the window. Locking or latching the window closed typically requires a plurality of connectors to be mounted along mating edges of the sash and the perimeter frame for engagement with one another. When securing a connector, for example a lock keeper, to a sash formed of hollow plastic members, fasteners are known to come loose from the sash over time. A reinforcement member can be mounted in the hollow plastic member forming the sash to span between an adjacent pair of lock keepers, however in regions where climate fluctuates considerably between different seasonal temperatures, the different rates of expansion and contraction of the reinforcement member and the plastic of the sash can cause the sash to bow and not fit properly within the perimeter frame.